1. Field of the Invention
The present invention relates to a triaxial heating cable, and, more particularly, to a triaxial heating cable system with a power controller.
2. Description of the Related Art
The conversion of electrical energy into heat has been utilized for a significant period of time. The direct conversion of electrical energy to heat was first described by English physicist James P. Joule. According to Joule's law, a conductor carrying a current generates heat at a rate proportional to the product of the resistance of the conductor and the square of the current. It is the use of this principal, of applying electrical energy to a distributed resistance, which provides heat to an area or volume that is in thermal contact with the distributed resistance. Distributed resistance electrical heaters are utilized in providing heat to surfaces such as walkways or pavement, thereby removing snow and ice.
Now referring to FIGS. 1–3 there is illustrated cross-sectional views of distributed resistive elements. In FIG. 1 there is shown a mineral insulated heating cable 10 including heater wires 12, insulation 14 and metallic sheath 16. Heater wires 12 traverse the length of heating cable 10 and are electrically resistive in nature. Insulation 14 is a mineral type insulation capable of withstanding significant temperatures and conducting heat from heater wires 12 to metallic sheath 16. Heat that originates with heater wires 12 is additionally passed through metallic sheath 16 to thereby heat any item in contact with metallic sheath 16. Resistive wires 12 are terminated at a distal end of the cable to thereby provide a completed electrical circuit to another end of heating cable 10.
In FIG. 2 there is shown a self limiting heating cable 20 including a first bus wire 22, a second bus wire 24, conductive polymer 26 and insulation/shield 28. The shape of self limiting heating cable 20 is often elliptical in nature in order to conduct and dissipate heat produced between bus wires 22 and 24. Bus wires 22 and 24 are conductive wires with relatively low resistance as compared to wires 12 of heating cable 10. Bus wires 22 and 24 provide an electrical potential difference therebetween that is conducted through conductive polymer 26. Heat thus generated is then thermally conducted in and through conductive polymer 26 and through insulation shield 28 to an item that is in contact with self-limiting heating cable 20. Conductive polymer 26 is typically a positive temperature coefficient material, which increases in resistance with the rise of temperature thereby self-limiting the temperature of heating cable 20.
Mineral insulated heating cable 30 is similar to cable 10 except that it has only one heater wire 32 surrounded by insulation 34 and metallic sheath 36. One end of heater wire 32 is connected with a source and another end of heater wire 32 is connected to a return line (not shown). A single conductor cable such as heating cable 30 requires that both ends be electrically connected in order to provide a circuit therethrough.
A problem with heating cables 10 and 30 are that the heat is generated in and at the surface of heater wires 12 and 32 thereby causing a high temperature area to exist in the center of cable 10 and 30 respectively. The high temperature area must be accommodated by an electrical insulating material that can efficiently remove the heat generated at resistive wires 12 and 32. The problem with self-limiting heating cable 20 is that the heat is mainly generated between bus wires 22 and 24 and not uniformly in conductive polymer 26. This is somewhat compensated for by an elliptical shape of such a cable to reduce the distance from the area in which heat is generated to an exterior surface of heating cable 20, but the shape requires a large radius to bend it about the major axis. Additionally, self-limiting heating cable 20 is typically very stiff and resists bending, thus making installation difficult.
The use of cable 10 requires a matching of the length of the cable with the resistivity of wires 12 to the potential power source to be applied to optimize the heat generated within cable 10. Cable 20 may be cut to any length and rely upon the self-limiting feature to prevent an overheating condition for cable 20. Cable 30 in addition to the disadvantages of cable 10 requires that both ends be terminated most likely away from the object being heated, thereby requiring access to each end of cable 30.
What is needed in the art is a heater cable that can be cut to length, be easy to install and provide an efficient heat distribution feature.